It is well known that some of the noise generated in machines is attributed to hydraulic noise which may be transmitted in various forms such as air-borne, fluid-borne, and/or structure borne. Many times, attempts are made to control hydraulic noise by enclosing the hydraulic system in an acoustical enclosure. However, this may not be feasible in many systems because some of the hydraulic components or the structures that they are mounted to are separated by a significant distance. In the various systems, one of the primary generators of hydraulic noise is the hydraulic pump. The hydraulic pump emits air-borne noise directly off the pump body, as well as structural vibrations to the pump mounting, the pump drive shaft and associated hydraulic lines. Likewise, the hydraulic pump also excites fluid-borne noise which is transmitted to valves, lines, and so forth and then to the structures of those components or the structures that they are mounted on. These structures then emit vibrations that create the largest portion of the overall air-borne noise attributed to the hydraulic system. Thus, it is obvious that the reduction of fluid-borne noise is a key to the reduction of noise generated by hydraulic systems.
Positive displacement hydraulic pumps, due to their geometry, port timing, and speed, inherently produce a flow ripple that excites the pressure waves that are known as fluid-borne noise. This is true of most, if not all, types of positive displacement piston, vane, or gear pumps or motors. For illustration purposes only, the structure and operation of a piston pump will be described hereafter. It is recognized that the same principles apply with respect to the other types of positive displacement pumps.
The total flow output of the hydraulic piston pump is geometrically proportional to the sum of the velocities of the individual pistons between the bottom dead center (BDC) and the top dead center (TDC) positions. The uneven delivery of fluid flow resulting from the sum of velocities not being constant is one of the inherent characteristics of a pump contributing to the flow ripple. A second source of flow ripple is due to pressure changes that occur at BDC when the pump is operating at some outlet pressure other than a low pressure equal to inlet pressure. When the piston reaches BDC, the piston cavity is at inlet pressure. As the piston is exposed to the high discharge pressure, flow from the outlet rushes into the piston cavity thus reducing the pump's total output flow. The amount and rate of flow change at BDC varies depending on the geometry of the cavities, the displacement of the pump, the porting configuration, the pump speed and the output pressure. Thus the flow ripple depends not only on the geometric sum of the piston velocities, but also on the pressure at which the pump is operating, the pump displacement, the pump porting, and the speed of the pump. By cancelling the flow ripple, the fluid-borne noise excited by the pump is eliminated along with the structure-borne noise and the air-borne noise that are associated with the hydraulic components or structures downstream thereof.
Various attempts have been made to reduce fluid-borne noise in hydraulic systems by installing various mufflers and/or dampers. Likewise, port timing is sometimes changed within the pump in an attempt to modify the pressure ripple. Even though some of these attempts have proven to be partially successful, they are normally only successful when operating within a given pressure, speed and displacement of the pump. However, when systems are being operated over wide ranges of speed, displacement and pressure, these earlier arrangements have proven to be inadequate. It is desirable, therefore, to provide a system that can effectively control the fluid-borne noise therein when operating in a large range of speeds, pressures and/or displacements.
The present invention is directed to overcoming one or more of the problems as set forth above.